Refactor/2479/simplify databackend contract

[blythed]

Description

• Only 1 type of query superduper.backends.base.query.Query
• Implementations based on superduper_<impl>.Databackend
• Simplifications to query API:
  • t.insert(...) instead of t.insert().execute() since we don't need to serialize insertions
  • t.update, t.delete deprecated
  • read queries have a simpler form t.filter(...).select(...).outputs(...), or t.like().(...) or t.(...).like()
  • t.get() to get one data point (eager)
  • t.ids() to get the ids (eager)
  • t.subset(ids) to subset a query
  • t.limit(n, offset=m) to get a chunk of data
• .execute() no longer returns a cursor, instead a simple list
• Remove the error prone t.column == x, replace with t['column'] == x
• Simpler serialization of "complex items" with q.dict()['documents']